1. Field of the Invention
The present invention relates to a line-type heater used as a heating source or the like in a toner fixing portion of a copying machine or an electrophotographic printer.
2. Description of the Related Art
Conventionally, such a line-type heater is generally well known. For example, as disclosed in JP-A-7-147180, a line-type heater has such a configuration that a heating resistance film is formed on the surface of an insulating substrate which is formed from a ceramic material and formed into a long strip so that the heating resistance film extends in a line in the longitudinal direction of the insulating substrate. The insulating substrate is attached to a stay member so that the back of the insulating substrate is in tight contact with the surface of the stay member. Thus, the heating resistance film can heat all over its length when a current is supplied to the heating resistance film on the surface of the insulating substrate from the both sides of the heating resistance film.
In addition, conventionally, to manufacture the line-type heater, such a method has been adopted as follows. That is, a raw substrate made from ceramic material and formed by putting a plurality of sheets of long strip-like insulating substrates side by side and integrating them with each other is formed. A heating resistance film is formed on the surface of this raw substrate at places corresponding to the respective insulating substrates, and then the raw surface is broken into pieces corresponding to the insulating substrates.
However, in such a line-type heater, it cannot be expected to generate a crack when the temperature of the heating resistance film becomes abnormally high temperature. Therefore, there is a fear that a more abnormally high temperature may be caused.
Therefore, conventionally, at least one small-diameter through hole is formed in the insulating substrate at a portion on its longitudinally way, so that the insulating substrate can crack at the portion of the through hole in case of abnormal temperature to thereby prevent the temperature from increasing more.
However, when at least one small-diameter through hole is formed in an insulating substrate at a portion on its longitudinal way so that the insulating substrate can crack at the portion of the through hole in case of abnormal temperature, the strength of the insulating substrate is reduced extremely in the portion where the through hole is formed because the insulating substrate is shaped in a long strip. Accordingly, there was a problem that when the insulating substrate was manufactured by breaking a raw substrate having a plurality of sheets of such insulating substrates put side by side and integrated with each other, or when the broken pieces of the insulating substrate were handled ordinarily, for example, when they carried, attached and so on, the broken pieces of insulating substrates were often snapped in their through hole portions.
There was another problem that when the insulating substrate was cracked due to abnormal temperature of its heating resistance film, the broken pieces of the heating resistance film contacted with and separated from each other repeatedly in their broken surfaces so as to generate sparks in those portions.